The present invention relates to a semiconductor device and, more particularly, to a bipolar transistor requiring a small area, having a small base resistance and particularly suitable for operation at a very high speed.
Referring to FIG. 1, showing a conventional bipolar transistor fabricated by a self-alignment technique, there are shown an n-type emitter region 11, a p-type base region 12, a heavily doped n-type collector region 5, a silicon dioxide film 14, a silicon dioxide film 15, an n-type polycrystalline silicon film 16, an emitter lead 17, a base (or base lead-out) lead 18 and a base electrode 19 formed of a p-type polycrystalline silicon film. The distance between the right-hand end of the p-type polycrystalline silicon film 19 and the right-hand end of the emitter region 11 must be not less than about 0.6 .mu.m since the alignment error resulting during the formation of a base electrode 19 of polycrystalline silicon film by using photolithographic technique must be taken into consideration. As a result, therefore, reduction of this distance is limited by the alignment error in photolithographic technique. The base electrode 19 is formed of only polycrystalline silicon.
A bipolar transistor having such a structure is disclosed in IEEE, Trans. Electron Der., pp. 2246-2254, ED-34, No. 11 (1987).
Since the size of the base electrode 19 of this conventional bipolar transistor is limited by the accuracy of aligning the base electrode 19 relative to the n-type emitter region 11 in a photolithographic process, it has been difficult to reduce the size of the base electrode 19. Even if the size of the base electrode 19 could be reduced, the resistance of the base electrode 19 increases because the width of the base electrode 19 surrounding the emitter region 11 is reduced. Consequently, current is locally concentrated in a portion of the base electrode 19 in contact with the base lead 18, which deteriorates the high-frequency characteristics of the bipolar transistor.
The emitter region 11 must be formed in a smaller area to form the bipolar transistor in a compact construction. However, if the base electrode 19 is formed to have a relatively large thickness, the resistance, in a vertical direction of the polycrystalline silicon film 16 formed on the emitter region 11 increases. Therefore, the base electrode 19 must be formed in a relatively small thickness to make the resistance of the polycrystalline silicon film 16 in a vertical direction relatively small. The sheet resistance of a highly doped p-type polycrystalline silicon film which is of 500 nm in thickness and which is doped to a maximum impurity concentration is about 50 .OMEGA./.quadrature.. The sheet resistance is inversely proportional to the thickness of the polycrystalline silicon film; a polycrystalline silicon film of a smaller thickness has a greater sheet resistance. Accordingly, if the thickness of the base electrode 19 is reduced to prevent the increase of the resistance, in a vertical direction, of the polycrystalline silicon film 16 formed on the emitter region 11, the base series resistance of a narrow portion of the base electrode 19 increases, thereby reducing the operating speed.